LinuxKPI: Add min_not_zero() to linux/minmax.h
It returns the minimum that is not zero, except both equals to zero.
Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz, emaste, ssaxena
Differential Revision: https://reviews.freebsd.org/D45450
Differential Revision: https://reviews.freebsd.org/D45451
(cherry picked from commit 07c7c41ca1e31c20c2e2d8fb68c148f521831e4d)